1. Field of the Invention
The present invention relates to a focus detecting apparatus and a control method thereof, and in particular to a focus detecting apparatus that performs auto focus detection and a control method thereof.
2. Description of the Related Art
Currently, general image capturing apparatuses such as still cameras and video cameras have an auto focus detection (AF) function. As the auto focus detection method, contrast detection method and phase-difference detection method are widely used. With the phase-difference detection method, the distance to a subject (or the amount of defocus) is detected from the phase difference between signals obtained from a pair of line sensors, using the principle of triangulation, and the position of a focusing lens of an image capturing lens is controlled based on the result of the detection, whereby the image capturing lens can focus on the subject.
Two specific configurations for implementing the phase-difference detection method are known: so-called TTL method in which light that has entered an image capturing lens is received by line sensors; and so-called external AF in which light that has entered via a route other than passing through an image capturing lens is received by line sensors. With the former method, the amount of defocus can be detected based on the phase difference between output signals from the line sensors, and with the latter method, the distance to a subject can be detected.
With the external AF method, an optical system that performs focus detection (optical system for focus detection) is provided at a position different from that of an optical system that forms an image of a subject to be captured (optical system for imaging). This causes parallax between the systems, so that the imaging area and the focus detection area do not match. Also, the phase difference between signals obtained from a pair of line sensors of an external AF sensor increases as the distance to the subject becomes shorter and decreases as the distance to the subject becomes longer. Due to the principle of the external AF method, it is necessary to dynamically optimize and select pixels (detection pixel portion) for use in phase difference detection from among a plurality of pixels included in the line sensors according to the distance to the subject. Accordingly, with respect to the external AF method, a method has been proposed in Japanese Patent Laid-Open No. 2003-98421 that determines an optimal focus detection area to use from a plurality of focus detection areas of a line sensor. Japanese Patent Laid-Open No. 2003-98421 discloses a technique in which when the focus detection area is switched based on the focal length (angle of view) of the zoom lens of the camera, a focus detection area to be selected is determined according to the result of focus detection obtained in a focus detection area positioned substantially in the center of an image capturing screen.
As described above, with the external AF method, in the case of a subject in a short distance, because the phase difference between signals obtained from a pair of line sensors is very large, it is necessary to provide more pixels in the individual line sensors in order to achieve highly accurate focus detection on the subject in a short distance. In contrast, in the case of a subject in a long distance, because there is little phase difference between signals, highly accurate focus detection is possible using a smaller number of pixels than in the case where focus detection is performed on a subject in a short distance. Accordingly, if focus detection is performed on a subject in a long distance by using the same number of pixels as that used to perform focus detection on a subject in a short distance, it will lead to over processing, and is therefore not desirable in terms of processing time and power consumption.